The measurement of relative elevation is an important element in almost any phase of a construction project. In the last few decades, the use of laser reference planes has grown as a way to satisfy this demand for accurate elevation information. The laser plane is created by a laser transmitter. A variety of methods are used to do this, but one of the most common is a rotating beam of light. The laser transmitter is typically placed at a known elevation. The plane of light is frequently set up to be level with respect to gravity so that all points on the laser plane are at the same elevation.
To use this laser plane to measure the elevation of a point, a worker will mount a laser receiver on a pole or a handle. The bottom of the pole/handle will rest on the point whose elevation will be measured. However, the pole/handle must be held plumb with respect to gravity. The laser receiver is positioned in the plane of laser light such that it indicates an on-grade position. The relative elevation can then be read as the length of the pole/handle between the point being measured and the on-grade position of the receiver. Many different pole/handle designs are available to make this job easier. Most provide an easy height adjustment and a scale that can be read at a convenient height for the worker.
While these methods are widely used, there is room for improvement. The pole/handle itself can be expensive and less rugged than users desire. Much care must be taken to assure accurate measurements. Some points on a jobsite are inaccessible for measurement by this method, so it can be seen that there are limitations to that methodology.
In addition to the above, the use of GPS receivers for survey and machine control is well known in the field of construction. However, due to technical limitations in GPS receivers, the vertical accuracy is at least a factor of two (2) worse than the accuracy in the plane parallel to the Earth's surface. Frequently, the vertical dimension is the most important, so there is an incentive to improve this.